One type of subsea wellhead assembly has a production tree mounted to a wellhead housing at the upper end of the well. In one type of production tree, referred to as a horizontal tree, a tubing hanger lands in the production tree. The tubing hanger supports a string of tubing and has an axial passage that receives the flow of well fluid. The axial passage has a lateral outlet between upper and lower ends of the axial passage. The lateral outlet registers with a lateral outlet in the production tree. At least one plug is secured in the axial passage above the lateral outlet to cause the well fluid to flow out the lateral outlet. The plug is retrievable and normally installed and retrieved with a wireline.
For safety reasons, at least two pressure barriers are located in the tree to prevent well fluid from flowing above the tubing hanger. The first pressure barrier is considered to be the wireline plug. In some installations, a tree cap is installed in the bore of the production tree above the tubing hanger to serve as the second pressure barrier. In other installations, a second wireline plug is installed in the tubing hanger passage above the first plug. The second or upper wireline plug is considered to be the second pressure barrier.
The axial passage in the tubing hanger contains a liquid before the upper plug is lowered in place above the lower plug. Normally, there is no outlet leading to the space in the passage between the plugs, thus the liquid will be trapped. During production, the well fluid flowing up the tubing can be at elevated temperatures because of the temperature of the producing formation. The elevated temperature can cause the liquid in the trapped fluid space to expand. The expansion would cause the pressure within the trapped fluid space to rise and could result in leakage across the seals of one or both of the wireline plugs. When the well is shut in, the temperature will drop because of the cool temperature of the sea water at the sea floor. The liquid in the trapped fluid space shrinks and the pressure drops. This reduction in pressure could cause sea water to be drawn across the seals of the upper plug. The cycling of temperature can damage the seals, reducing the effectiveness of the plugs as being pressure barriers.